Aircraft main engines not only provide propulsion for the aircraft, but in many instances may also be used to drive various other rotating components such as, for example, generators and pumps. The main engines may also be used to supply compressed air to the aircraft's environmental control system, which may be used to supply temperature-controlled air to both the aircraft cabin and to electronic equipment within the aircraft.
When an aircraft is on the ground and its main engines are not being used, an alternative power source may be used to supply power for ground support operations. In addition, during some ground support operations, an external supply of cooling and heating air may be used to supply temperature-controlled air to the cabin and aircraft equipment. For some type of aircraft ground support applications, most notably military aircraft ground support applications, a ground cart may be used to supply electrical power to the aircraft and compressed air to an air conditioning system module. The air conditioning module in turn may supply temperature-controlled air to the aircraft cabin and the aircraft's electronic equipment.
One particular air conditioning system module that may be used during aircraft ground support operations receives high temperature (e.g., ≧300° F.) compressed air supplied by the ground cart, and conditions the compressed air to a predetermined temperature. The air conditioning system module may be used in at least two modes, a cooling mode, to supply cool air, or a heating mode, to supply warm air. To do so, the air conditioning system module may include a primary heat exchanger, a condenser, a moisture separator, and one or more cooling turbines. Typically, this air conditioning system module is designed so that when it is operating in the cooling mode it will supply cool air at a predetermined desired temperature for a given, predetermined design ambient temperature. For example, the system may be designed to supply cooling air at a temperature no higher than 55° F. when the ambient temperature is 125° F. Thus, when actual ambient temperature is below the design ambient temperature, the air conditioning system may supply cooling air that is less than 55° F.
In some instances, supplying air to an aircraft at less than 55° F. may not be desirable. Moreover, in some instances, it may be desirable to supply heating air to an aircraft at temperatures of up to 200° F. Hence, the air conditioning system module may include a bypass flowpath for use in the heating mode. The bypass flowpath allows a portion of the high temperature compressed air to flow around the primary heat exchanger, and back into the cooled compressed air stream that is exhausted from the primary heat exchanger. For example, a valve may be installed in a bypass duct, and the valve may be positioned to control hot compressed air bypass flow rate, to thereby control the temperature exiting the primary heat exchanger, and in turn controlling the temperature of the air being supplied by the air conditioning system module.
Although the above-described system and method for controlling air temperature to an aircraft during ground support operations, in both a cooling mode and a heating mode is effective, it suffers certain drawbacks. For example, it can be difficult to consistently control the temperature of the air by feeding a portion of the hot compressed air back into the compressed air that has been cooled. In addition, the cost of the air conditioning system module may be increased because high temperature ductwork and a high temperature valve may be needed to implement the compressed air bypass flow path.
Hence, there is a need for a system and method of providing conditioned air to an aircraft environmental control system during ground support operations that does not use hot compressed air to control air supply temperature when ambient temperature is below the maximum design temperature and/or is less costly than presently known systems and methods. The present invention addresses one or more of these needs.